Zeolites are porous crystalline or quasi-crystalline aluminosilicates constructed of repeating SiO4 and AlO4 tetrahedral units. These units are linked together to form frameworks having regular intra-crystalline cavities and channels of molecular dimensions. Numerous types of synthetic zeolites have been synthesized and each has a unique framework based on the specific arrangement of its tetrahedral units. By convention, each framework type is assigned a unique three-letter code (e.g., “AFX”) by the International Zeolite Association (IZA).
AFX frameworks incorporate both an AFT-cage structure and a GME-cage structure. When combined, the AFT and GME-cages form the unique AFX crystalline structure. Conventional synthesis techniques for AFX zeolites involve the use of Na-GME zeolites to directly provide the GME portion of the crystal, and a structure directing agent (SDA), also referred to as a “template” or “templating agent” to form the AFT portion of the crystal. SDAs are typically complex organic molecules that guide or direct the molecular shape and patter of the zeolite's framework. Generally, the SDA serves to position hydrated silica and alumina as a mold around which the zeolite crystals form. After the crystals are formed, the SDA can be removed from the interior structure of the crystals, leaving a molecularly porous aluminosilicate cage.
Zeolites have numerous industrial applications including the catalytic treatment of exhaust gas from combustion of hydrocarbon fuels, such as internal combustion engines, gas turbines, coal-fired power plants, and the like. To improve catalytic performance, zeolites are frequently loaded with a transition metal, such as copper. In one example, a metal loaded zeolite can catalytically reduce the concentration of nitrogen oxides (NOx) in the exhaust gas via a selective catalytic reduction (SCR) process.
Sodium (Na) has two detrimental impacts on the synthesis and the final product. First, the presence of Na can lead to various impurities in the final product. These impurities can at times dominate the crystalline batch or may contain rod-like crystals which may pose health hazards. Additionally, for SCR catalysts, the presence of residual Na can impact both the exchange of Cu and the durability of the final catalyst product to high temperatures in excess of 800° C., where the formation of Na aluminates can collapse the structure. Also, the presence of Na typically results in lower SAR materials that are more sensitivity to hydrothermal aging. Accordingly, there remains a need to avoid these issues associated with the presence of alkali in the synthesis. This invention satisfies these needs amongst others.